Generally, a membrane switch sheet provides a switch matrix structure which is widely used in key input apparatuses such as membrane-type keyboards.
A membrane-type keyboard is a keyboard in which a thin conductive film is placed under keys. In a membrane method, ON or OFF is determined based on the contact between sheets using the elastic force of the membrane.
Silicon rubber has been known to be placed under the keys of a keyboard which uses such a membrane switch.
When a key placed on top of the silicon rubber is pressed, a specific portion of the silicon rubber exerts pressure against the top of a contact (switch) of the membrane switch sheet.
At this time, the contacts (switches) of the upper and lower films of the membrane switch sheet form a connection. Accordingly, an operation of pressing the key is transmitted using an electrical signal conducted through the contacts.
When the key is no longer being pressed, the contacts (switches) of the upper and lower films of the membrane switch sheet become separated from each other by the elastic (restoring) force of the membrane switch sheet and the elastic (restoring) force of the silicon rubber. Accordingly, since an electrical signal is no longer being conducted through the contacts, it is recognized that the operation of pressing the key has stopped.
The membrane switch sheet includes three thin polyester films. An example of the membrane switch sheet will be described below with reference to FIGS. 1 to 4.
FIG. 1 is a diagram illustrating a conventional membrane switch sheet 71.
Referring to FIG. 1, the membrane switch sheet 71 includes a thin film-type insulating spacer material 72 provided with a plurality of through holes 75 and a pair of flexible thin insulation films 73 and 74 stacked on and beneath the insulating spacer material 72, respectively.
The insulating spacer material 72 and the thin insulation films 73 and 74 may be formed of, for example, polyester films.
One thin insulation film 73 includes second electrodes 76, resistors 77 and lines 78, which are formed on the surface thereof which comes into contact with the spacer material 72, using a pattern formation method. The second electrodes 76 are placed opposite the respective through holes 72. The lines 78 are connected to the lead portions of the second electrodes 76, the lead portions of the resistors 77 and the other ends of the resistors 77.
The lines 80 of the other thin insulation film 74 include lines 80a configured to connect first electrodes 79 in series and lines 80b configured to connect the first electrodes 79 to an external element.
The electrodes 76 and 79 and the lines 78 and 80 may have a two-layer structure including a first conductive layer made of silver and a layer configure to have a low resistance value, or a two-layer structure including a second layer formed on a first layer having a low resistance value in order to prevent movement.
The switch pattern having the first layer and the second layer may be formed by mixing an ink base, such as polyester or epoxy, with silver powder or conductive carbon, printing a mixture on the film, and then performing heating and sintering. The printed surface operates as an electric conductor having a resistant property that enables electrical signals to be transmitted.
The signal lines formed on the films through the printing process using the conductive ink can achieve a desired conductivity (a proper resistance value) by adjusting the mixing ratio of a conductive medium, such as silver powder or conductive carbon, to the ink base.
In FIG. 1, the membrane switch sheet 71 includes the thin insulation film 74 separated from the thin insulation film 73 and the spacer material 72. In the membrane switch sheet 71, when the top surface of the thin insulation film 74 on which an electrode 79 is formed is pressed downward by a key, the corresponding electrode 79 is connected to the opposite electrode 76 on the thin insulation film 73 by the deformation of the thin insulation film 74. When the downward force is released, the contact between the electrodes 76 and 79 is removed by the elastic force of the thin insulation film 74.
FIGS. 2 to 4 are diagrams illustrating another conventional membrane switch sheet.
FIG. 2 shows the lower film of the membrane switch sheet, FIG. 3 shows the center film of the membrane switch sheet, and FIG. 4 shows the upper film of the membrane switch sheet. The three films are stacked one on top of another, thus forming the membrane switch sheet. The membrane switch sheet constitutes the upper portion of a switch matrix in order to form a switch matrix-type key input apparatus.
The lower film 23 of the membrane switch sheet shown in FIG. 2 is formed by printing the lines of the switch matrix in the column scan direction on the top surface of the film 23 using conductive ink. Here, patterning is performed twice because two types of resistors Rs and Rr have to be patterned. The resistors Rr having a high resistance value and the resistors Rs having a low resistance value have to be separately patterned. Furthermore, the center film 24 of the membrane switch sheet shown in FIG. 3 includes a plurality of holes which will become switches. The center file 2 functions as an insulator with only portions for switches removed therefrom. Finally, the upper film 25 of the membrane switch sheet shown in FIG. 4 is formed by printing the lines of the switch matrix in the row scan direction on a film using conductive ink having a resistance value Rs.
In the membrane switch sheet composed of three sheets of film, that is, the lower film 23, the center film 24 and the upper film 25, when weight is applied to the upper film (i.e., when a key signal is input through the switching operation of the switch matrix, a contact is formed between the upper film 25 and the lower film 23, thus turning on a switch. When the weight is released (i.e., when the switching operation of the switch matrix is stopped), the switch is turned off by the restoring force of the film itself.
The switch pattern having the lower film 23 and upper film 25 of the membrane switch sheet is formed by mixing silver powder or conductive carbon with an ink base such as polyester or epoxy, printing a mixture on the film and then performing heating and sintering. The printed surface functions as an electric conductor having a resistant property that enables electrical signals to be transmitted. As described above, the resistors Rs are formed on the upper film 25 through the printing process using the conductive ink, so that the resistors having conductivity can be used as contacts. Furthermore, the resistors Rs having a low resistance value and the resistors Rr having a high resistance value are also formed on the lower film 23 using conductive ink through a first printing process and a second printing process, respectively, so that the resistors having conductivity are used as the contacts.
Accordingly, the signal lines formed on the upper film 25 and the lower film 23 through the printing processes using the conductive ink can achieve a desired conductivity (a proper resistance value) by adjusting the mixing ratio of the conductive medium, such as silver powder or conductive carbon, to the ink base.
As described above, the conventional membrane switch sheet is implemented by forming the patterns, corresponding to switch wiring having a low resistance value, such as the resistors Rs, in the lines of the column scan direction and then forming the resistors Rr having a high resistance value near the contacts of the switches.
However, the conventional membrane switch sheet is problematic in that the manufacturing process is complicated and the manufacturing price is high because it requires conductive ink (the resistors Rr and Rs) to be printed on the lower film 23 (i.e., the same polyester film) in two separate processes and requires heating and sintering to be performed.